The same problem was posted in the FB group. The poster also mentioned that the TB icon did not appear in the panel (KDE Plasma).
Personally, I did not experience any problems with the new version of TB. Not using many add-ons anyway.

With the new version 60.0, not all add-ons are enabled. That might be the reason why the add-ons do not work.

From TB release notes:
‘IMPORTANT: Add-ons not marked as compatible with Thunderbird 60 by their authors will be disabled (this can be reverted via preference extensions.strictCompatibility)’

As for the language. Could it be that the language pack was not yet updated?